Process for producing high mesophase content pitch fibers

ABSTRACT

An improved process for producing carbon fibers from pitch which has been transformed, in part, to a liquid crystal or so-called &#39;&#39;&#39;&#39;mesophase&#39;&#39;&#39;&#39; state. According to the process, the mesophase content of fibers spun from such pitch is increased before the fibers are thermoset and carbonized by vacuum distillation of the non-mesophase content of the fibers.

United States Patent 1191 Schulz Nov. 11, 1975 (5 PROCESS FOR PRODUCINGHIGH 3.552.922 1/1971 Shikklwfl ct 264/29 MESOPHASE CONTENT PITCH FIBERS1 E 6 00 8t 11 Inventor f' Schulz. ir i k. 3.629.379 12/1971 0mm 264/29OhlO 3.634.220 H1973 GOan 204/164 4 3.718.493 2/i973 J00 et ul. 106/273R [73] Assgnee Carb'de Cmpmamm New 3.767.741 10/1973 Toyoguchi et61.....-

264/29 York 3.787.541 1/1974 Grindstuff et al 264/29 [22] Filed: Dec.26. I972 I Prim/1r E.\'aminer-]ay H. W00 [21] Appl' M8382 Attorney.Agent, or Firm-J. S. Piscitello 52 us. c1. 1. 264/102; 264/29;2o4/lilgk4lf7; [57] ABSTRACT [5 1] Int CL B24c'25/00 An improved processfor producing carbon fibers [58] Field DIG 19 from pitch which has beentransformed. in part, to u 6-, 704/164 liquid crystal or so-calledmesophase state. According to the process. the mcsophase content offibers [56] References Cited spun from such pitch is increased beforethe fibers arc UNITED STATES PATENTS Bernhurdt et ul. 264/176 Fthermoset and carbonized by vacuum distillation of the non-mesophasecontent of the fibers.

36 Claims. N0 Drawings PROCESS FOR PRODUCING I-IIGII MESOPI-IASE CONTENTPITCH FIBERS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to an improved process for producing carbon fibersfrom pitch which has been transformed, in part, to a liquid crystal orso-called mesophase" state. More particularly, this invention relates toan improved process for producing carbon fibers from pitch of this typewherein the mesophase content of fibers spun from such pitch isincreased before the fibers are thermoset and carbonized by vacuumdistillation of the non-mesophase content of the fibers.

2. Description of the Prior Art As a result of the rapidly expandinggrowth of the aircraft, space and missile industries in recent years, aneed was created for materials exhibiting a unique and extraordinarycombination of physical properties. Thus, materials characterized byhigh strength and stiffness, and at the same time of light weight, wererequired for use in such applications as the fabrication of aircraftstructures, re-entry vehicles, and space vehicles, as well as in thepreparation of marine deep-submergence pressure vessels and likestructures. Existing technology was incapable of supplying suchmaterials and the search to satisfy this need centered about thefabrication of composite articles.

One of the most promising materials suggested for use in composite formwas high strength, high modulus carbon textiles, which were introducedinto the market place at the very time this rapid growth in theaircraft, space and missile industries was occurring. Such textiles havebeen incorporated in both plastic and metal matrices to producecomposites having extraordinary high-strengthand high-modulus-to-weightratios and other exceptional properties. However, the high cost ofproducing the high strength, high modulus carbon textiles employed insuch composites has been a major deterrent to their widespread use, inspite of the remarkable properties exhibited by such composites.

One recently proposed method of providing high modulus, high strengthcarbon fibers at low cost is described in copending application Ser. No.239,490, entitled l-ligh Modulus, High Strength Carbon Fibers ProducedFrom Mesophase Pitch." Such method comprises first spinning acarbonaceous fiber from a carbonaceous pitch which has been transformed,in part, to a liquid crystal or so-called mesophase state, thenthermosetting the fiber so-produced by heating the fiber in anoxygen-containing atmosphere for a time sufficient to render it totallyinfusible, and finally carbonizing the thermoset fiber by heating in aninert atmosphere to a temperature sufficiently elevated to removehydrogen and other volatiles and produce a substantially all-carbonfiber. The carbon fibers produced in this manner have a highly orientedstructure characterized by the presence of carbon crystallitespreferentially aligned parallel to the fiber axis, and are graphitizablematerials which when heated to graphitizing temperatures develop thethree-dimensional order characteristic of polycrystalline graphite andgraphiticlike properties associated therewith, such as high density andlow electrical resistivity.

Since carbonaceous fibers drawn from pitches having a high mesophasecontent can be thermoset in less time than carbonaceous fibers drawnfrom pitches having a lower mesophase content, it is desirable to employpitches of high mesophase content in such process. However, because thespinning of mesophase-containing pitches becomes increasingly difficultas the mesophase content of the pitch increases, and must be done athigher and higher temperatures, the fibers are usually prepared frompitches having a mesophase content of only from about 40 percent byweight to about percent by weight.

SUMMARY OF THE INVENTION In accordance with the present invention, ithas now been discovered that pitch fibers having a high mesophaasecontent can be prepared from pitch fibers of lower mesophase contentwhich have been spun from pitches of the type described inaforementioned copending application Ser. No. 239, 490, Le, carbonaceouspitches which have been transformed, in part, to a liquid crystal orso-called mesophase state, by subjecting the fibers to low-pressure heattreatment so as to volatilize at least a portion of the non-mesophasecontent of the fiber; and that the so treated fibers can be converted byfurther heat treatment into carbon fibers having a high young's modulusof elasticity and high tensile strength. The invention takes advantageof the differences in molecular weight and volatility between themolecules of the mesophase portion of the fiber and the molecules of thenon-mesophase portion to effect removal of the non-mesophase portion andproduce a fibrous residue of higher mesophase content. The non-mesophasemol ecules are of lower molecular weight than the higher molecularweight mesophase molecules and are preferentially volatilized from thefiber during the initial heat treatment to produce a fiber of highermesophase content. This non-mesophase material can be substantiallycompletely removed by the distillation or only partially, depending uponthe relative amounts of mesophase and non-mesophase material present inthe fibers, the diameter of the fibers, the heat treatment temperature,the pressure employed during the heat treatment, and the duration of theheat treatment. Surprisingly, it has been found from weight loss data onthe fibers, together with solubility and polarized ligh microscopystudies, that the increase in mesophase content of the heat-treatedfibers is not due solely to volatilization of the nonmesophase material,but results, in part, from conversion of this non-mesophase tomesophase.

The fibers produced in this manner have a high degree of preferredorientation of their molecules parallel to the fiber axis and can beconverted by heat treatment into carbon fibers having a high Youngsmodulus of elasticity and high tensile strength. The carbon fibersso-produced have a highly oriented structure characterized by thepresence of carbon crystallites preferentially aligned parallel to thefiber axis, and when heated to graphitizing temperatures develop thethree-dimensional order characteristic of polycrystalline graphite andgraphitic-like properties associated therewith, such as high density andlow electrical resistance. At all stages of their development from theas-drawn condition to the graphitized state, the fibers arecharacterized by the presence of large oriented graphitizable domainspreferentially aligned parallel to the fiber axis, with the fibers aftervacuum distillation, however, containing a lesser amount ofnon-mesophase material than before such distillation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS When natural or syntheticpitches having an aromatic base are heated at a temperature of about350C.-450C., either at constant temperature or with gradually increasingtemperature, small insoluble liquid spheres begin to appear in the pitchand gradually increase in size as heating is continued. When examined byelectron diffraction and polarized light techniques, these spheres areshown to consist of layers of oriented molecules aligned in the samedirection. As these spheres continue to grow in size as heating iscontinued, they come in contact with one another and gradually coalescewith each other to produce larger masses of continuous aligned layers.Eventually, substantially the entire pitch coalesces and takes on thesuperficial appearance of a mosaic structure where, however, thetransition from one oriented region to another occurs smoothly andcontinuously through gradual curving lamellar regions rather thanthrough sharp boundaries between uniform areas of oriented lamellae.

The highly oriented, optically anisotropic, insoluble material producedby treating pitches in this manner has been given the term mesophase,"and pitches containing such material are known as mesophase pitches.Such pitches, when heated above their softening points, are mixtures oftwo immiscible liquids, one the optically anisotropic, orientedmesophase portion in either spherulite or coalesced form, and the otherthe isotropic nonmesophase portion. The term mesophase" is derived fromthe Greek mesos or intermediate and indicates the pseudo-crystallinenature of this highly-oriented, optically anisotropic material.

C arbonaceous pitches having a mesophase content of from about 40percent by weight to about 70 percent by weight can easily be spun intofibers which can subsequently be converted by heat treatment into carbonfibers having a high Youngs modulus of elasticity and high tensilestrength. Although fibers can also be spun from pitches having amesophase content in excess of about 70 percent by weight, e.g., up toabout 90 percent by weight, these pitches are exceedingly difficult towork with because of their high softening temperatures, and fibers canonly be spun from such pitches at elevated temperatures where thepitches readily undergo polymerization and/or coking.

In order to obtain highly oriented carbonaceous fibers capable of beingheat treated to produce fibers having the three-dimensional ordercharacteristic of polycrystalline graphite from carbonaceous pitcheshaving a mesophase content of from about 40 percent by weight to about90 percent by weight, however, it is not only necessary that such amountof mesophase be present, but also that it be present in the form oflarge, homogeneous, coalesced regions. Pitches which polymerize veryrapidly develop small or stringy mesophase regions rather than largecoalesced regions and are unsuitable. Likewise, pitches which do notform homogeneous coalesced regions of mesophase are unsuitable. Thelatter phenomenon is caused by the presence of non-mesophase insolubles(which are either present in the original pitch or which develop onheating) which are enveloped by the coalescing mesophase and serve tointerrupt the homogeneity and uniformity of the coalesced domains.

Another requirement is that the pitch be nonthixotropic under theconditions employed in the spinning of the pitch into fibers i.e., itmust exhibit a Newtonian or plastic flow behavior so that the viscositycoefficient is independent of the shear rate of the pitch during thespinning process. When such pitches are heated to a temperature wherethey exhibit a viscosity of from about l0 poises to about 200 poises,uniform fibers may be readily spun therefrom. Thixotropic pitches, onthe other hand, which do not exhibit Newtonian or plastic flow behaviorwhen attempts are made to spin fibers therefrom, but rather undergochanges in apparent viscosity, do not permit uniform fibers to be spuntherefrom which can be converted by further heat treatment into fiberscapable of developing the threedimensional order characteristic ofpolycrystalline graphite.

carbonaceous pitches having a mesophase content of from about 40 percentby weight to about percent by weight can be produced in accordance withknown techniques, as disclosed in aforementioned copending applicationSer. No. 239, 490, by heating a carbonaceous pitch in an inertatmosphere at a temperature above about 350C. for a time sufficient toproduce the desired quantity of mesophase. By an inert atmosphere ismeant an atmosphere which does not react with the pitch under theheating conditions employed, such as nitrogen, argon, zenon, helium andthe like. The heat ing period required to produce the desired mesophasecontent varies with the particular pitch and temperature employed, withlonger heating periods required at lower temperatures than at highertemperatures. At 350C, the minimum temperature generally required toproduce mesophase, at least one week of heating is usually necessary toproduce a mesophase content of about 40 percent. At temperatures of fromabout 400C. to 450C, conversion to mesophase proceeds more rapidly, anda 50 percent mesophase content can usually be produced at suchtemperatures within about 1-40 hours. Such temperatures are preferredfor this reason. Temperatures above about 500C. are undesirable, andheating at this temperature should not be employed for more than about 5minutes to avoid conversion of the pitch to coke.

The degree to which the pitch has been converted to mesophase canreadily be determined by polarized light microscopy and solubilityexaminations. Except for certain non-mesophase insolubles present in theoriginal pitch or which, in some instances, develop on heating, thenon-mesophase portion of the pitch is readily soluble in organicsolvents such as quinoline and pyridine, while the mesophase portion isinsoluble. 1 In the case of pitches which do not develop non-mesophaseinsolubles when heated, the insoluble content of the heat treated pitchover and above the insoluble content of the pitch before it has beenheat treated is due to conversion of the pitch to mesophase. in the caseof pitches which do develop non-mesophase insolubles when heated, theinsoluble content of the heat treated pitch over and above the insolublecontent of the pitch before it has been heat treated is not solely dueto the conversion of the pitch to mesophase, but also representsnon-mesophase insolubles which are produced along with the mesophaseduring the heat treatment. Pitches which contain such non-mesophaseinsolubles (either present in the original pitch or developed byheating) in amounts sufficient to prevent the development of homogeneouscoalesced mesophase regions are unsutiable for use in the presentinvention, as noted above. Generally, ptiches which contain in excess ofabout 2 percent by weight of such materials are unsuitable. The presenceor absence of such homogeneous coalesced mesophase regions, as well asthe presence or absence of non-mesophase insolubles, can be visuallyobserved by polarized light microscopy examination of the pitch (see,e.g., Brooks, J. D., and Taylor, G. H., The Formation of SomeGraphitizing Carbons," Chemistry and Physics of Carbon, Vol. 4, MarcelDekker, Inc., New York, 1968, pp. 243-268; and Dubois, J., Agache, C.,and White, J. L., The Carbonaceous Mesophase Formed in the Pyrolysis ofGraphitizable Organic Materials," Metallography 3, 337-369, 1970). Theamounts of each of these materials may also be visually estimated inthis manner. 1 The per cent of quinoline insolubles (0.1.) of a givenpitch is determined by quinoline extraction at 75C. The percent ofpyridine insolubles (P.l.) is determined by Soxhlet extraction inboiling pyridine. (l C.,). 2 The insoluble content of the untreatedpitch is generally less than 1 percent (except for certain coal tarpitches) and consists largely of coke and carbon black found in theoriginal pitch.

Aromatic base carbonaceous pitches having a carbon content of from about92 percent by weight to about 96 percent by weight and a hydrogencontent of from about 4 percent by weight to about 8 percent by weightare generally suitable for producing mesophase pitches which can beemployed to produce fibers capable of being heat treated to producefibers having the threedimensional order characteristic ofpolycrystalline graphite. Elements other than carbon and hydrogen, suchas oxygen, sulfur and nitrogen, are undesirable and should not bepresent in excess of about 4 percent by weight. The presence of morethan such amount of extraneous elements may disrupt the formation ofcarbon crystallites during subsequent heat treatment and prevent thedevelopment of a graphitic-like structure within the fibers producedfrom these materials. In addition, the presence of extraneous elementsreduces the carbon content of the pitch and hence the ultimate yield ofcarbon fiber. When such extraneous elements are present in amounts offrom about 0.5 percent by weight to about 4 percent by weight, thepitches generally have a carbon content of from about 92-95 percent byweight, the balance being hydrogen.

Petroleum pitch, coal tar pitch and acenaphthylene pitch, which arewell-graphitizing pitches, are preferred starting materials forproducing the mesophase pitches which are employed to produce the fibersof the instant invention. Petroleum pitch, of course, is the residuumcarbonaceous material obtained from the distillation of crude oils orthe catalytic cracking of petroleum distillates. Coal tar pitch issimilarly obtained by the distillation of coal. Both of these materialsare commercially available natural pitches in which mesophase can easilybe produced, and are preferred for this reason. Acenaphthylene pitch, onthe other hand, is a synthetic pitch which is preferred because of itsability to produce excellent fibers. Acenaphthylene pitch can beproduced by the pyrolysis of polymers of acenaphthylene as described byEdstrom et al. in U.S. Pat. 3,574,653.

Some pitches, such as fluoranthene pitch, polymerize very rapidly whenheated and fail to develop large coalesced regions of mesophase, andare, therefore, not suitable precursor materials. Likewise, pitcheshaving a high non-mesophase insoluble content in organic solvents suchas quinoline or pyridine, or those which develop a high non-mesophaseinsoluble content when heated, should not be employed as startingmaterials, as explained above, because these pitches are incapable ofdeveloping the homogeneous regions of coalesced mesophase which arenecesary to produce highly oriented carbonaceous fibers capable ofdeveloping the three-dimensional order characteristic of polycrystallinegraphite. For this reason, pitches having a quinoline-insoluble orpyridine-insoluble content of more than about 2 percent by weight(determined as described above) should not be employed, or should befiltered to remove this material before being heated to producemesophase. Preferably, such pitches are filtered when they contain morethan about I percent by weight of such insoluble material. Mostpetroleum pitches and synthetic pitches have a low insoluble content andcan be used directly without such filtration. Most coal tar pitches, onthe other hand, have a high insoluble content and require filtrationbefore they can be employed.

As the pitch is heated at a temperature between 350C. and 500C. toproduce mesophase, the pitch will, of course, pyrolyze to a certainextent and the composition of the pitch will be altered, depending uponthe temperature, the heating time, and the composition and structure ofthe starting material. Generally, however, after heating a carbonaceouspitch for a time sufficient to produce a mesophase content of from about40 percent by weight to about percent by weight, the resulting pitchwill contain a carbon content of from about 94-96 percent by weight anda hydrogen content of from about 4-6 percent by weight. When suchpitches contain elements other than carbon and hydrogen in amounts offrom about 0.5 percent by weight to about 4 percent by weight, themesophase pitch will generally have a carbon content of from about 92-95percent by weight, the balance being hydrogen.

After the desired mesophase pitch has been prepared, it is spun intofibers by conventional techniques, e.g., by melt spinning, centrifugalspinning, blow spinning, or in any other known manner. As noted above,in order to obtain highly oriented carbonaceous fibers capable ofdeveloping the three-dimensional order characteristic of polycrystallinegraphite the pitch must contain large homogeneous regions of coalescedmesophase and be nonthixotropic under the conditions employed in thespinning.

The temperature at which the pitch is spun depends, of course, upon thetemperature at which the pitch exhibits a suitable viscosity. Since thesoftening temperature of the pitch, and its viscosity at a giventemperature, increases as the mesophase content of the pitch increases,the mesophase content should not be permitted to rise to a point whichraises the softening point of the pitch to excessive levels. For thisreason, pitches having a mesophase content of more than about 70 percentare usually not employed. Pitches containing a mesophase content ofabout 40 percent by weight usually have a viscosity of about 200 poisesat about 250C. and about l0 poises at about 300C, while pitchescontaining a mesophase content of about 70 percent by weight exhibitsimilar viscosities at about 390C. and 440C., respectively. Within thisviscosity range, fibers may be conveniently spun from such pitches at arate of from about 20 feet per minute to about feet per minute and evenup to about 3000 feet per minute. Preferably, the pitch employed has amesophase content of from about 50 percent by weight to about 65 percentby weight and exhibits a viscosity of from about 30 poises to about 60poises at temperatures of from about 340C. to about 380C. At suchviscosity and temperature, uniform fibers having diameters of from aboutmicrons to about microns can be easily spun. As previously mentioned,however. in order to obtain the desired fibers, it is important that thepitch be nonthixotropic and exhibit Newtonian or plastic flow behaviorso that the viscosity coefficient is independent of the shear rate ofthe pitch during the spinning of the fibers.

The carbonaceous fibers produced in this manner are highly orientedgraphitizable materials having a high degree of preferred orientation oftheir molecules parallel to the fiber axis. By graphitizable is meantthat these fibers are capable of being converted thermally (usually byheating to a temperature in excess of about 2500C., e.g., from about2500C. to about 3000C.) to a structure having the three-dimensionalorder characteristic of polycrystalline graphite.

The fibers produced in this manner, of course, have the same chemicalcomposition as the pitch from which they were drawn, and like such pitchcontain from about 40 percent by weight to about 90 percent by weightmesophase. When examined under magnification by polarized light andscanning electron microscopy techniques, large fibrillar-shaped domainsof mesophase interspersed with large elongated nonmesophase regions canbe seen distributed throughout the fiber, giving the fibers theappearance of a minicomposite. These fibrillar mesophase domains arehighly oriented and preferentially aligned parallel to the fiber axis.Characteristically, these domains have diameters in excess of 5,000 A,generally from about 10,000 A to about 40,000 A, and because of theirlarge size are easily observed when examined by conventional polarizedlight microscopy techniques at a magnification of 1000. (The maximumresolving power of a standard polarized light microscope having amagnification factor of 1000 is only a few tenths of a micron 1 micron10,000 A] and anisotropic domains having dimensions of 1000 A or lesscannot be detected by this technique.) a

After the fibers have been spun, as hereinbefore described, they areheated under reduced pressure so as to volatilize at least a portion ofthe non-mesophase portion of the fiber. As previously stated, theinvention takes advantage of the differences in molecular weight andvolatility between the molecules of the mesophase portion of the fiberand the molecules of the non-mesophase portion to effect removal of thenon-mesophase portion and produce a fibrous residue of high mesophasecontent. As has been noted, the non-mesophase molecules are of lowermolecular weight than the higher molecular weight mesophase moleculesand are preferentially volatilized from the fiber during the heattreatment to produce a fiber of higher mesophase content. Thisnon-mesophase material can be substantially completely removed by thedistillation or only partially. depending upon the relative amounts ofmesophase and non-mesophase present in the fibers, the diameter of thefibers, the heat treatment temperature, the pressure employed during theheat treatment, and the duration of the heat treatment. The extent ofwhich non-mesophase has been volatilized can readily be determined bythe loss in weight which the fibers undergo during the heat treatment.

Removal of the non-mesophase portion of the fibers is effected byheating the fibers under a pressure of less than about 100 microns Hg,preferably less than 30 microns Hg, at a temperature and for a timesufficient to volatilize as much of the non-mesophase material from thefibers as desired. The temperature employed must be sufficiently high toeffect the desired degree of volatilization but must not, of course,exceed the temperature at which the fibers will soften or distort, orthe temperature at which sintering of fibers in contact with each otheroccurs. Higher temperatures permit more complete volatilization of thenon-mesophase material in a given time than do lower temperatures. Byemploying sufficiently high temperatures for an appropriate time, it ispossible to substantially completely remove the entire non-mesophasecontent of the fibers.

A minimum temperature of at least 250C. is generally necessary tovolatilize non-mesophase material from the fibers. Temperatures inexcess of 400C. may cause melting of the fibers and should be avoided.Preferably, temperatures of from about 300C. to about 390C. areemployed.

The time required to effect removal of the non-mesophase portion of thefibers will, of course, vary with such factors as the relative amountsof mesophase and non-mesophase material present in the fibers, thediameter of the fibers, the heat treatment temperature, and the pressureemployed during the heat treatment. Relatively thick fibers and/orfibers having a relatively high non-mesophase content require longerheating times to effect the removal than do thinner fibers or fibershaving a lower non-mesophase content. Likewise, the use of highertemperatures and/or lower pressures permit a given amount ofnon-mesophase material to be removed in shorter periods of time than ispossible at lower temperatures and/or higher pressures. Removal of atleast 5 percent by weight of the non-mesophase content of the fibers canusually be effected by heating at an appropriate temperature within fromabout 5 minutes to about 30 minutes. Removal of from about 10 percent byweight to about 40 percent by weight of the non-mesophasc contentusually requires more protracted heating times, e.g., from about 0.5hour to about 100 hours or more.

The fibers produced in this manner, like their precursors, arecharacterized by the presence of large oriented graphitizable domainspreferentially aligned parallel to the fiber axis, with the fibers aftervacuum distillation, however, containing a lesser amount of nonmesophasematerial than before such distillation. By further heat treatment, thesefibers can be converted into carbon fibers having a high Youngs modulusof elasticity and high tensile strength.

While heat-treated fibers containing in excess of about percent byweight mesophase are, at times, sufficiently infusible to permit them tobe carbonized without any prior thermosetting treatment, fiberscontaining less than about 85 percent by weight mesophase require somethermosetting before they can be carbonized. (Evidently, the fiberscontaining more than 85 percent by weight mesophase are sufficientlyreinforced by their fibrillar structure to allow them to be carbonizeddirectly without any prior thermosetting treatment.) In any event,because of the higher ratio of mesophase to non-mesophase of the heattreated fibers compared to their precursors, they can be thermoset, atany given temperature, in shorter periods of time than said precursors.

Thermosetting of the fibers is readily effected by heating the fibers inan oxygen-containing atmosphere for a time sufficient to render themtotally infusible. The oxygen-containing atmosphere employed may be 9pure oxygen or an oxygen-rich atmosphere. Most conveniently, air isemployed as the oxidizing atmosphere.

The time required to effect thermosetting of the fibers will, of course,vary with such factors as the particular oxidizing atmosphere, thetemperature employed, the diameter of the fibers, the particular pitchfrom which the fibers are prepared, and the mesophase content of thefibers. Generally, however, thermosetting of the fibers can be effectedin relatively short periods of time, usually in from about 4 minutes toabout 50 minutes.

The temperature employed to effect thermosetting of the fibers must, ofcourse, not exceed the temperature at which the fibers will soften ordistort. The maximum temperature which can be employed will thus dependupon the particular pitch from which the fibers were spun, and themesophase content of the fibers. The higher the mesophase content of thefiber, the higher will be its softening temperature, and the higher thetemperature which can be employed to effect thermosetting. At highertemperatures, of course, fibers of a given diameter can be thermoset inless time than is possible at lower temperatures. Fibers having a lowermesophase content, on the other hand, require relatively longer heattreatment at somewhat lower temperatures to render them infusible.

A minimum temperature of at least 250C. is generally necessary toeffectively thermoset the heat-treated fibers produced in accordancewith the invention. Temperatures in excess of 400C. may cause meltingand/or excessive burn-off of the fibers and should be avoided.Preferably, temperatures of from about 325C. to about 390C. areemployed. At such temperatures, thermosetting can generally be effectedwithin from about 4 minutes to about 50 minutes. Since it is undesirableto oxidize the fibers more than necessary to render them totallyinfusible, the fibers are generally not heated for longer than about 50minutes, or at temperatures in excess of 400C.

After the fibers have been thermoset, the infusible fibers arecarbonized by heating in an inert atmosphere, such as that describedabove, to a temperature sufficiently elevated to remove hydrogen andother volatiles and produce a substantially all-carbon fiber. Fibershaving a carbon content greater than about 98 percent by weight cangenerally be produced by heating to a temperature in excess of about1000C., and at temperatures in excess of about 1500C., the fibers arecompletely carbonized.

Usually, carbonization is effected at a temperature of from about l000C.to about 2000C., preferably from about l500C. to about l900C. Generally,residence times of from about 0.5 minute to about 25 minutes, preferablyfrom about 1 minute to about 5 minutes, are employed. While moreextended heating times can be employed with good results, such residencetimes are uneconomical and, as a practical matter, there is no advantagein employing such long periods.

In order to ensure that the rate of weight loss of the fibers does notbecome so excessive as to disrupt the fiber structure, it is preferredto heat the fibers for a brief period at a temperature of from about700C. to about 900C. before they are heated to their final carbonizationtemperature. Residence times at these temperatures of from about 30seconds to about 5 minutes are usually sufficient. Preferably, thefibers are heated at a temperature of about 700C. for about one-halfminute and then at a temperature of about 900C. for

10 like time. In any event, the heating rate must be controlled so thatthe volatilization does not proceed at an excessive rate.

In a preferred method of heat treatment, continuous filaments of thefibers are passed through a series of heating zones which are held atsuccessively higher temperatures. Several arrangements of apparatus canbe utilized in providing the series of heating zones. Thus, one furnacecan be used with the fibers being passed through the furnace severaltimes and with the temperature being increased each time. Alternatively,the fibers may be given a single pass through several furnaces, witheach successive furnace being maintained at a higher temperature thanthat of the previous furnace. Also, a single furnace with severalheating zones maintained at successively higher temperatures in thedirection of travel of the fibers, can be used.

The carbon fibers produced in this manner have a highly orientedstructure characterized by the presence of carbon crystallitespreferentially aligned parallel to the fiber axis, and are graphitizablematerials which when heated to graphitizing temperatures develop thethree-dimensional order characteristic of polycrystalline graphite andgraphite-like properties associated therewith, such as high density andlow electrical resistivity.

If desired, the carbonized fibers may be further heated in an inertatmosphere, as described hereinbefore, to a still higher temperature ina range of from about 2500C. to about 3300C., preferably from about2800C. to about 3000C., to produce fibers having not only a high degreeof preferred orientation of their carbon crystallites parallel to thefiber axis, but also by a structure characteristic of polycrystallinegraphite. A residence time of about I minute is satisfactory, althoughboth shorter and longer times may be employed, e.g., from about 10seconds to about 5 minutes, or longer. Residence times longer than 5minutes are uneconomical and unnecessary, but may be employed ifdesired.

The fibers produced by heating at a temperature above about 2500C.,preferably above about 2800C., are characterized as having thethree-dimensional order of polycrystalline graphite. Thisthree-dimensional order is established by the X-ray diffraction patternof the fibers, specifically by the presence of the (H2) cross-latticeline and the resolution of the (i0) band into two distinct lines, andl0l The short arcs which constitute the (00!) bands of the pattern showthe carbon crystallites of the fibers to be preferentially alignedparallel to the fiber axis. Microdensitometer scanning of the (002) bandof the exposed X-ray film indicate this preferred orientation to be nomore than about 10, usually from about 5 to to about 10 (expressed asthe full width as half maximum of the azimuthal intensity distribution).Apparent layer size (L,) and apparent stack height (L of thecrystallites are in excess of 1000 A and are thus too large to bemeasured by X-ray techniques. The interlayer spacing (d) of thecrystallites, calculated from the distance between the corresponding(00!) diffraction arcs, is no more than 3.37 A, usually from 3.36 A to3.37 A.

EXAMPLE 1 A commercial petroleum pitch was employed to produce a pitchhaving a mesophase content of about 59 percent by weight. The precursorpitch had a density of I24 grams/cc., a softening temperature of C. and

1 1 contained about 1 percent by weight pyridine insolubles (P. l. wasdetermined by Soxhlet extraction in boiling pyridine). Chemical analysisshowed a carbon content of about 93%. a hydrogen content of about 6%, asulfur content of about 1% and 0.15% ash.

The mesophase pitch was produced by heating the precursor petroleumpitch at a temperature of about 400C. for about hours under a nitrogenatmosphere. After heating, the pitch contained 59.8 percent by weightpyridine insolubles, indicating that the pitch had a mesophase contentof close to 59 percent.

A portion of this pitch was transferred to an extrusion cylinder andspun into fiber by applying pressure to the pitch with an argur whilethe molten pitch was extruded through a pin-hole orifice (diameter 0.015inch) at the bottom of the extruder at a rate of between 200 to 400feet/minute. The filament passed through a nitrogen atmosphere as itleft the extruder orifice and was then taken up by a reel. Aconsiderable quantity of fiber 35 microns in diameter was produced inthis manner at a temperature of 380C.

A portion of the as-drawn fiber was heated at a temperature of 315C. for64 hours under a pressure of 20 microns Hg. The fiber showed a loss inweight of 1 1.5 percent as a result of the heat treatment.

Surprisingly, the fiber contained 90 percent by weight pyridineinsolubles after heat treatment, indicating that the fiber had amesophase content of about 90 percent. Based on the mesophase content ofthe asdrawn fiber and the loss in weight during the heat treatment, themesophase content of the fiber should have been only 68 percent. Thisindicated that a portion of the non-mesophase present in the as-drawnfiber had been converted to mesophase during the heat treatment.Polarized light microscopy studies of the fiber also indicated asubstantial increase in mesophase content as a result of the heattreatment.

The heat treated fiber essentially fully retained the integrity of theas-drawn fiber and showed no serious disruptions in the fiber surface.

Another portion of the as-drawn fiber was heated at a temperature of360C. for a total of 2.5 hours under a pressure of 20 microns Hg. Aftereach half hour period, the fiber was removed from the oven, cooled, andweighed. The weight loss of the fiber during each half hour period isindicated below:

Time, minutes What is claimed is:

l. A process for producing a pitch fiber having a high mesophase contentwhich comprises spinning a carbonaceous fiber from a nonthixotropiccarbonaceous pitch containing from 40 percent by weight to 70 percent byweight mesophase, said mesophase being present in the form of large,homogeneous. coalesced regions, and heating the spun fiber under apressure of less than 100 microns Hg at a temperature and for a timesufficient to volatilize at least a portion of the nonmesophase contentof the fiber and produce a fiber of higher mesophase content.

2. A process as in claim 1 wherein the fiber is heated at a temperatureof from 250C. to 400C.

3. A process as in claim 1 wherein the fiber is heated at a temperatureof from 300C. to 390C.

4. A process as in claim 1 wherein the fiber is heated at a temperatureand for a time sufficient to volatilize at least 5 percent by weight ofthe non-mesophase content of the fiber.

5. A process as in claim 4 wherein the fiber is heated at a temperatureof from 250C. to 400C.

6. A process as in claim 4 wherein the fiber is heated at a temperatureof from 300C. to 390C.

7. A process as in claim 1 wherein the fiber is heated at a temperatureand for a time sufficient to volatilize from 10 percent by weight to 40percent by weight of the non-mesophase content of the fiber.

8. A process as in claim 7 wherein the fiber is heated at a temperatureof from 250C. to 400C.

9. A process as in claim 7 wherein the fiber is heated at a temperatureof from 300C. to 390C.

10. A process as in claim 1 wherein the fiber is heated under a pressureof less than 30 microns Hg.

11. A process as in claim 10 wherein the fiber is heated at atemperature of from 250C. to 400C.

12. A process as in claim 10 wherein the fiber is heated at atemperature of from 300C. to 390C.

13. A process as in claim 10 wherein the fiber is heated at atemperature and for a time sufficient to volatilize at least 5 percentby weight of the non-mesophase content of the fiber.

14. A process as in claim 13 wherein the fiber is heated at atemperature of from 250C. to 400C.

15. A process as in claim 13 wherein the fiber is heated at atemperature of from 300C. to 390C.

16. A process as in claim 10 wherein the fiber is heated at atemperature and for a time sufficient to volatilize from 10 percent byweight to 40 percent by weight of the non-mesophase content of thefiber.

17. A process as in claim 16 wherein the fiber is heated at atemperature of from 250C. to 400C.

18. A process as in claim 16 wherein the fiber is heated at atemperature of from 300C. to 390C.

19. A process as in claim 1 wherein the pitch contains from 50 percentby weight to 65 percent by weight mesophase.

20. A process as in claim 19 wherein the fiber is heated at atemperature of from 250C. to 400C.

21. A process as in claim 19 wherein the fiber is heated at atemperature of from 300C. to 390C.

22. A process as in claim 19 wherein the fiber is heated at atemperature and for a time sufficinet to volatilize at least 5 percentby weight of the non-mesophase content of the fiber.

23. A process as in claim 22 wherein the fiber is heated at atemperature of from 250C. to 400C.

24. A process as in claim 22 wherein the fiber is heated at atemperature of from 300C. to 390C.

25. A process as in claim 19 wherein the fiber is heated at atemperature and for a time sufficient to volatilize from 10 percent byweight to 40 percent by weight of the non-mesophase content of thefiber.

26. A process as in claim 25 wherein the fiber is heated at atemperature of from 250C. to 400C.

27. A process as in claim 25 wherein the fiber is heated at atemperature of from 300C. to 390C.

28. A process as in claim 19 wherein the fiber is heated under apressure of less than 30 microns Hg.

33. A process as in claim 31 wherein the fiber is heated at atemperature of from 300C. to 390C.

34. A process as in claim 28 wherein the fiber is heated at atemperature and for a time sufficient to volatilize l percent by weightto 40 percent by weight of the non-mesophase content of the fiber.

35. A process as in claim 34 wherein the fiber is heated at atemperature of from 250C. to 400C.

36. A process as in claim 34 wherein the fiber is heated at atemperature of from 300C. to 390C.

UNITED STATES PATENT OFFICE g 1 of 2 CERTIFICATE OF CORRECTION Patent:No. 3,919! 576 Dat d Novemberll, 1975 Inventor(s) David chulz It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 2, line 15, "aase" should read ase Column 2, line 25, "young's"should read Young's Column 2, line 45, "ligh" should read light Column3, line 69, insert a comma after "fibers".

Column 4, line ll, "threedi-" should read three-di- Column 4, line 66,"unsutiable" should read unsuitable Column 4, line 67, "ptiches" shouldread pitches Column 5, line 15, "(ll5C should read (ll5C)- Column 7,line 61, "of" should read to UNITED STATES PATENT OFFICE Page 2 of 2CERTIFICATE OF CORRECTION Patent No. 919,376 Dated November 11, 1975Inventor(x) David A. Schulz It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 10, line 54, "to" (second occurrence) should be deletedu Column12, line 52, "sufficinet" should read sufficient --o Column 14, line 5,"10 10" should read from 10 Signed and Scalcd this smm Day Of Novemberma [SEAL] Arrest:

RUTH MASON 00mm w. BANNER Arresting Omar Commissioner afhmm andTrademarks

1. A POCESS FOR PRODUCING A PITCH FIBER HAVING A HIGH MESOPHASE CONTENTWHICH COMPRISES SPINNING A CABONACEOUS FIBER FROM A NONTHIXOTROPICCARBONACEOUS PITCH CONTAINING FROM 40 PERCENT BY WEIGHT TO 70 PERCENT BYWIGHT MESOPHASE, SAID MESOPHASE BEING PRESENT IN THE FORM OF LARGE,HOMOGENEOUS, ALESCED REGIONS, AND HEATING THE SPUN FIBER UNDER APRESSURE OF LESS THAN 100 MICRONS HG AT A TEMPERATURE AND FOR A TIMESUFFICIENT TO VOLATILIZE AT LEAST APORTION OF THE NON-MESOPHASE CONTENTOF THE FIBE AND PRODUE A FIBE HIGHE MESOPHASE CONTENT.
 2. A process asin claim 1 wherein the fiber is heated at a temperature of from 250*C.to 400*C.
 3. A process as in claim 1 wherein the fiber is heated at atemperature of from 300*C. to 390*C.
 4. A process as in claim 1 whereinthe fiber is heated at a temperature and for a time sufficient tovolatilize at least 5 percent by weight of the non-mesophase content ofthe fiber.
 5. A process as in claim 4 wherein the fiber is heated at atemperature of from 250*C. to 400*C.
 6. A process as in claim 4 whereinthe fiber is heated at a temperature of from 300*C. to 390*C.
 7. Aprocess as in claim 1 wherein the fiber is heated at a temperature andfor a time sufficient to volatilize from 10 percent by weight to 40percent by weight of the non-mesophase content of the fiber.
 8. Aprocess as in claim 7 wherein the fiber is heated at a temperature offrom 250*C. to 400*C.
 9. A process as in claim 7 wherein the fiber isheated at a temperature of from 300*C. to 390*C.
 10. A process as inclaim 1 wherein the fiber is heated under a pressure of less than 30microns Hg.
 11. A process as in claim 10 wherein the fiber is heated ata temperature of from 250*C. to 400*C.
 12. A process as in claim 10wherein the fiber is heated at a temperature of from 300*C. to 390*C.13. A process as in claim 10 wherein the fiber is heated at atemperature and for a time sufficient to volatilize at least 5 percentby weight of the non-mesophase content of the fiber.
 14. A process as inclaim 13 wherein the fiber is heated at a temperature of from 250*C. to400*C.
 15. A process as in claim 13 wherein the fiber is heated at atemperature of from 300*C. to 390*C.
 16. A process as in claim 10wherein the fiber is heated at a temperature and for a time sufficientto volatilize from 10 percent by weight to 40 percent by weight of thenon-mesophase content of the fiber.
 17. A process as in claim 16 whereinthe fiber is heated at a temperature of from 250*C. to 400*C.
 18. Aprocess as in claim 16 wherein the fiber is heated at a temperature offrom 300*C. to 390*C.
 19. A process as in claim 1 wherein the pitchcontains from 50 percent by weight to 65 percent by weight mesophase.20. A process as in claim 19 wherein the fiber is heated at atemperature of from 250*C. to 400*C.
 21. A process as in claim 19wherein the fiber is heated at a temperature of from 300*C. to 390*C.22. A process as in claim 19 wherein the fiber is heated at atemperature and for a time sufficinet to volatilize at least 5 percentby weight of the non-mesophase content of the fiber.
 23. A process as inclaim 22 wherein the fiber is heated at a temperature of from 250*C. to400*C.
 24. A process as in claim 22 wherein the fiber is heated at atemperature of from 300*C. to 390*C.
 25. A process as in claim 19wherein the fiber is heated at a temperature and for a time sufficientto volatilize from 10 percent by weight to 40 percent by weight of thenon-mesophase content of the fiber.
 26. A process as in claim 25 whereinthe fiber is heated at a temperature of from 250*C. to 400*C.
 27. Aprocess as in claim 25 wherein the fiber is heated at a temperature offrom 300*C. to 390*C.
 28. A process as in claim 19 wherein the fiber isheated under a pressure of less than 30 microns Hg.
 29. A process as inclaim 28 wherein the fiber is heated at a temperature of from 250*C. to400*C.
 30. A process as in claim 28 wherein the fiber is heated at atemperature of from 300*C. to 390*C.
 31. A process as in claim 28wherein the fiber is heated at a temperature and for a time sufficientto volatilize at least 5 percent by weight of the non-mesophase contentof the fiber.
 32. A process as in claim 31 wherein the fiber is heatedat a temperature of from 250*C. to 400*C.
 33. A process as in claim 31wherein the fiber is heated at a temperature of from 300*C. to 390*C.34. A process as in claim 28 wherein the fiber is heated at atemperature and for a time sufficient to volatilize 10 10 percent byweight to 40 percent by weight of the non-mesophase content of thefiber.
 35. A process as in claim 34 wherein the fiber is heated at atemperature of from 250*C. to 400*C.
 36. A process as in claim 34wherein the fiber is heated at a temperature of from 300*C. to 390*C.